An asymmetric mirror is a planar layered optical device exhibiting asymmetry in reflectance of light incident from opposing sides, while its transmittance is symmetric. Asymmetric mirrors are useful in applications such as specialized Fabry-Perot interferometer systems, as described in Yu. V. Troitskii, Optics Spectrosc. 98, 125 (2005). Asymmetric mirrors have been realized using smooth thin metal films on planar dielectric substrates, using corrugated metal films (gratings), and using multi-layer thin-film stacks. The optical characteristics of these mirrors, such as reflectance asymmetry and the associated bandwidth are typically constrained to a narrow range, due to a limited choice of materials. In particular, their reflection asymmetry ΔR is limited to 3% or less at optical wavelengths, and limited to 2% or less if invariance of ΔR across a broad wavelength range is required.
The general relations describing the energy balance in a two-way mirror areT1+R1+A1=1andT2+R2+A2=1,where T, R and A are the mirror transmittance, reflectance and losses (i.e., absorption and/or scattering), respectively, and the subscripts 1 and 2 specify the direction of light-incidence on the mirror. An asymmetric mirror is defined as a two-way mirror with reflection asymmetry and transmittance symmetry, i.e.,ΔR=R1−R2≠0  (1)andΔT=T1−T2=0  (2)
These two conditions imply that the material layers composing the mirror are not symmetrical with respect to light incident from opposing sides of the mirror and that there are energy losses or gains as the light interacts with the mirror (e.g., due to scattering or absorption).
One known structure for an asymmetric mirror is a thin metal film deposited on a dielectric substrate, embedded in a uniform dielectric. For example, a simple realization of asymmetric mirror known in the art is produced by depositing a thin silver film on a glass substrate, embedded in a vacuum. FIG. 1A is a graph illustrating for this type of asymmetric mirror the dependence of calculated values of ΔR on the film thickness for several different wavelengths in the visible wavelength range. The graph shows that the maximum reflection asymmetry ΔR is 3% for a film thickness of 20 nm and wavelength of 500 nm. However, ΔR drops to 1.5% at 850 nm, i.e., there is a 50% variation in ΔR across the visible wavelength range. This variation of ΔR is not desirable for many practical applications. A smaller variation in ΔR can be obtained by reducing the film thickness, but only by reducing the maximum reflection asymmetry ΔR to 2% or less. For example, FIG. 1B is a magnified view of the graph of ΔR versus film thickness showing that, for a film thickness around 8 nm, ΔR varies by about 10% across the visible wavelength range. The reflection asymmetry ΔR, however, is reduced to 2%. This behavior shown in FIGS. 1A and 1B is typical to most metal films which are reflective in the visible and near-infrared (NIR) portions of the spectrum.
Clearly, it would be an advance in the art of optical devices to provide an asymmetric mirror with a significantly stronger reflection asymmetry over a broader range of wavelengths. In particular, it would be valuable for certain technological applications to increase the magnitude of ΔR significantly above 3% while simultaneously reducing the wavelength-dependent variation of ΔR significantly below 10%.